1. Field of the Invention
The present invention relates to the Microwave Landing System. More particularly the present invention relates to detecting, demodulating and decoding the Differential Phase Shift Keying (DPSK) transmissions of function identification and data transmissions associated with the international standard Microwave Landing System (MLS).
2. Description of the Prior Art
The Microwave Landing System (MLS) is an internationally standardized means to provide precision approach and landing guidance to properly equipped user aircraft. The MLS signal format as illustrated in FIGS. 1-3 consists of a series of guidance and data functions transmitted in a time-division-multiplex (TDM) signal format on a single one of 200 channels in the microwave C-band (503-5090 Mhz). The signal format is radiated by ground stations located at the airport and received by special equipment within the user aircraft.
Each MLS function slot is identified by a unique digital code transmitted as part of the function preamble using DPSK encoding. Each digital bit is 64 usec wide, for an effective digital data rate of 15,625 bits/sec.
The function preamble for all MLS functions is divided into at least three sections, as shown in FIG. 4. The first 13 bit period (832 usec) consists of an unmodulated carrier which is equivalent to a string of binary zeros encoded into DPSK. This is followed by a five bit digital Barker Code (11101) for time synchronization. Finally, a unique seven bit function identification code, consisting of five data and two parity bits, is sent to identify the ensuing MLS function.
Many classical texts on communications theory such as Communication Systems and Techniques. by Schwartz, M., Bennett, W. R., and Stein, S., McGraw --Hill Book Co., New York, 1966, illustrate the simple, straightforward means of DPSK decoding shown in FIG. 5. In this approach, a delayed version of the received signal is used as the reference oscillator in a coherent detector. The coherent detector output is proportional to the phase difference between this reference and the current received signal, i.e., between the current and previous DPSK bits. The actual numerical output of the coherent detector depends on the total electrical phase of the intermediate frequency (IF) signal, w.sub.if T, over the period, T, of a single DPSK bit, where w.sub.if is the angular frequency of the IF signal in radians/sec and T is measured in seconds.
As noted in Communication Systems and Techniques (cited above) and elsewhere, this technique is applicable only in cases where the frequency error in w.sub.if is small. If the frequency error, .epsilon..sub.if, is not small, the total phase (w.sub.if +.epsilon..sub.if)T can vary significantly from the expected result. In the extremes, the output of the coherent detector in FIG. 5 could be zero (when (w.sub.if +.epsilon..sub.if)T=.pi./2) or the state of the information bits could be reversed (when (w.sub.if +.epsilon..sub.if)T=.pi.).
In the MLS application, even a perfect receiver could experience frequency errors which exceed these limits, due to ground station frequency stability and aircraft motion doppler. This is discussed in Annex 10 to the Convention on International Civil Aviation, Volume 1, Chapter 3.11, ICAO, Montreal, Oct. 1987, as well as Minimum Operational Performance Standards for Microwave Landing System Airborne Receiving Equipment, DO-177, Change 2, RTCA, Washington, D.C., Sept. 1986. Thus, the classical approach to DPSK demodulation will not produce the desired performance under typical MLS operating conditions.
An implementation which has been successfully used in MLS receivers is shown in FIG. 6. In this approach, the input signal is used to drive a phased-locked carrier regeneration loop which provides an unmodulated local oscillator signal as a phase reference. The coherent detector output is then converted from DPSK to binary information by means of a simple digital circuit. The key feature of this previously demonstrated implementation is that the phase tracking is performed at the IF frequency, before demodulation of the DPSK information. This implementation has been very successful in MLS applications, but it requires additional hardware to implement the phase locked loop.
Previous patents in MLS signal processing such as U.S. Pat. Nos. 4,489,326 to Studenny and 4,017,862 to Wild, have centered on the decoding of the proportional angle guidance information by means of a variety of microprocessor-aided techniques. U.S. Pat. No. 4,926,186 by Kelly and La Berge and assigned to the same assignee as the present invention provides a hardware intensive computation architecture which is appropriate for applications requiring sampled data rates in excess of those easily handled in software. The present invention extends the use of a microprocessor--or digital signal processor--aiding to the phase demodulation process. Use of the described implementation allows the entire MLS decoding process to be performed within an existing microprocessor or computer, eliminating the need for relatively costly and relatively unreliable phase-locked loops for carrier regeneration. The present invention can be utilized in highly reliable MLS receiver architectures, including the Integrated Communications, Navigation, Identification (ICNI) electronics now being designed for advanced fighter aircraft and the Military Microwave Landing System Avionics Program.